Traveling wave tube with fast wave interaction means



y 3, 1966 RQH. PANTELL 3,249,792

TRAVELING WAVE TUBE WITH FAST WAVE INTERACTION MEANS I FIGJ Filed April10' 1961- FIG. 5 42 79;

INVENTOR. RICHARD H. PANTELL ATTORNEY United States Patent s 249 792TRAVELING WAVETUBE WITH FAST WAVE INTERACTION MEANS Richard H. Pantell,Palo Alto, Calif., assignor to Varian The present invention relates ingeneral to microwave tubes cap able of operating in the millimeter waveband and in particular to fast wave tubes employing electrons whichtraverse a periodic path.

Cumulative interaction between an electron beam and an electromagneticcircuit field can be obtained by having the phase velocity of thecircuit wave approximately equal to the beam velocity. Such a circuitwave is usually termed a slow wave, since the phase velocity is lessthan the free space velocity of light whereas in the ordinary waveguidethe phase velocity is greater than or equal to the velocity of light. Aslow wave circuit may be obrtained by means of a conducting helix or byperiodic loading of a waveguide. The characteristics of a slow wavecircuit make it difficult to achieve interaction with the slow wavecircuit in the millimeter wavelength region, because the electromagneticfield strength is greatest at the conducting surface and decays inmagnitude approximately in an exponential form away from the surface andthe periodicity of the slow wave structure is less than the freespacewavelength. The decay of electromagnetic field strength away from theconducting surface means a decreased beam coupling impedance unless thebeam is compressed into an area extremely close to the metallic walls ofthe waveguide. This results in problems of beam interception and heatdissipation. The necessity for having periodic loading which is lessthan the free-space wavelength introduces difficult slow wave circuitfabrication problems at millimeter wavelengths.

One approach which eliminates the above objections to the use of slowwave circuits for generating millimeter wavelengths is to injectelectrons into a longitudinal magnetic field with some initialtransverse motion. The electrons will rotate in a helical-beam path atthe cyclotron frequency in the transverse plane. If an electromagneticwave which is polarized in the transverse plane and which oscillates atthe cyclotron frequency is present, a cumulative energy exchange betweenthe electron and the wave will occur. There are two aspects to this typeinteraction: the synchronism condition between the electromagnetic waveand the electron beam and the bunching or discrimination phenomenon ofthe electron beam. The synchronism condition requires some relationshipamong the parameters of the system so that a cumulative interaction canoccur between an electron beam and a fast electromagnetic wave. Thebunching or discrimination phenomenon involves the manner in which aninitially unbunched beam of electrons may transfer energy to a circuitwave; in other words, the means by which electrons which enter in anaccelerating field do not absorb as much energy as the electrons in thedecelerating field deliver. A tube structure utilizing interactionbetween a periodic beam and a fast wave circuit is taught in myco-pending application U.S. Serial No. 32,762 entitled Traveling WaveInteraction Device filed May 31, 1960, now U.S. Patent 3,183,- 399,granted May 11, 1965. The present application is directed to animprovement over the structure taught in the aforementioned co-pendingapplication.

It is therefore the object of this invention to provide an improvedmicrowave tube incorporating electron beam interaction with fast waves.

The main feature of this invention is to produce a traveling wave tubetype electron tube having the necessary focusing structure to produce aperiodic beam tra- 10 of the tube.

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jectory whereby the electron beam and a two wire electromagnetic wavecircuit may cumulatively interact.

Another feature of the present invention is the use of a parallel planemagnetron using cross fields to provide a cumulative interaction betweenan electron beam pursuing a cycloidal path and an electromagnetic wave.

Other features and advantages of the present invention will becomeapparent upon a perusal of the specification taken in connection withthe accompanying drawings wherein:

FIG. 1 is a sectional view, in part diagrammatic, of a fast waveelectron tube apparatus employing a feature of the present invention,

FIG. 2 is a cross-sectional view of FIG. 1 taken along line 22 in thedirection of the arrows,

FIG. 3 shows an w-B diagram showing interaction between the electronbeam and an electromagnetic wave,

FIG. 4 shows in schematic form how the bunching of electrons occurs in arectangular waveguide, and

FIG. 5 is a sectional view in part diagrammatic, of a linear magnetronemploying the features of the present invention.

Referring now to FIGS. 1 and 2 there is shown a high frequency,traveling Wave type tube in which an electron beam traversing a helicalpath in a DC. longitudinal magnetic field interacts with a suitablewave. More specifically, a canted cathode emitter 1 is disposed at oneend of a cylindrical glass casing 12 and is heated to an operatingtemperature by heater elements and supplied with the necessary operatingpotentials from a source (not shown) via heater leads generally shown at2.

An anode is defined by apertured ring 3 provided for drawing theelectrons from the canted cathode emitter 1 at an angle, and formingthem into a beam. The beam emerging from the anode passes into an axialmagnetic field at the aperture in anode 3 at an angle where the beam isgiven an angular velocity. The beam entering the traveling wave tubeportion of the structure is matched to magnetic focusing structure 4 forproducing and maintaining a helical path at the cyclotron frequency.After passing through the focusing structure 4 the beam is collected bya collector 5, hermetically sealed at its periphery to the glass casing12.

Wave energy which it is desired to amplify is fed into the input sectionof the tube via a parallel transmission line 6 operating in the dominantmode. The input wave energy is propagated through the waveguide section6 of the tube forming a beam field interaction space 10 of the tubeapparatus. In this beam interaction space 10 the transverse electricfields of the circuit mode interact with the spiraling electron beams toproduce amplification of the wave energy by cumulative interaction. Theamplified wave energy on parallel line 6 is then extracted from the tubeat 7 and conducted to any suitable load (not shown).

Suitable operating potentials are applied to the anode member 3 withrespect to the cathode emitter 1. A power supply (not shown) is providedto insure suitable operating potential. The high cathode to anode D.C.potential is held off via a glass insulator 9 of the tube envelope 12.The power supply is preferably grounded at the positive end thereof suchthat the tube body may be operated at ground potential therebyminimizing the risk to operating personnel.

The particular magnetic focusing utilized in the structure of FIGS. 1and 2 is a solenoid 4 which provides a longitudinal magnetic fieldwithin the interaction area An adjustable power supply is provided (notshown) for the solenoid 4 so that the ratio of transverse tolongitudinal motion of the electron beam can be adjusted.

In operation of the present structure, a condition of synchronismbetween the beam and electromagnetic field is obtained so that theelectron beam 6 sees the same phase of the electromagnetic field E allalong its trajectory. Further, bunching of the electrons occurs alongthe beam path so that the initially unbunched beam of electrons maytransfer energy to the circuit wave.

For a circuit wave propagating as elwFW/m the synchronism condition isfor and z=ia t, Where =D.C. angular velocity of the beam and n: aninteger, a' constant D.C. longitudinal velocity of the beam and c=freespace velocity of light. Therefore, the synchronous frequency w is:

The minus sign corresponds to a beam velocity and circuit wavespropagating in the same direction. The plus sign is for a phase velocityopposite to longitudinal direction of the beam motion.

Before bunching occurs, electrons are distributed uniformly on thesurface of a cylinder rotating at the cyclotron frequency. Under such acondition there will be no net energy exchange between the electron beam6 and the electromagnetic fields E. There are as many electrons in anaccelerating field as there are in a retarding field, therefore, it isnecessary to accomplish R.F. bunching. In a strong D.C. longitudinalmagnetic field, electron motion in the transverse plane is inhibited bythe presence of the RR field. It is only along the longitudinaldirection that the electrons can move with relative ease.

For an explanation of the bunching phenomenon see FIG. 4. For a TB wavethe only component of RF. field producing a longitudinally directedforce is H -H in the transverse plane interacts with the DC. azimuthalelectron velocity to provide longitudinal bunching. Electrons which are180 apart in the azimuthal position in a transverse plane are moved inopposite longitudinal directions as shown by the dotted arrows.Eventually electrons which are separated by 180 are bunched A /2 apart.After bunching has occurred the electrons are in the form of a helixwith a pitch equal to the guide wavelength A and rotating in ahelical-beam path at the cyclotron frequency. It is then possible tohave the majority of the electrons in a position and moving in adirection so as to deliver a net energy to the circuit. Energyinterchange occurs between the transverse motion of the electrons andthe transverse R.F. electric field. It is important for the helical-beamtube to have most of the beam energy in the transverse motion as thebunching force is proportional to the tranverse velocity, v,, and thenumber of interaction intervals is inversely proportional to thelongitudinal velocity, Z

It is desirable to have the pitch of the beam pursuing its helical pathsmall, to allow more interaction between the electrons and theelectromagnetic circuit.

Bunching analogous to that described will occur between theelectromagnetic field of FIGS. 1 and 2 and the helical-beam path ofelectrons.

An appropriate w-B diagram is illustrated at FIG. 3 to show howinteraction occurs between the helical electron beam 5 and theelectromagnetic wave H The solid line in FIG. 3 is the usual dispersioncurve for a transmission line. The dash lines are obtained by letting/z' )z, which expresses the DC. relationship between angular andlongitudinal motion. Thus, an effective propagation constant of the form(ipqi /z' ifi) results, where fi==w/v p=an integer corresponding to thenumber of angular variations of the RF. field. Curve I corresponds to afield propagating in the negative 2 and positive directions and curve IIcorresponds to a field propagating in the positive z and positivedirections.

A tube of this type will support amplification in a fast wave tube notonly in both the forward and backward directions but if the tube currentis high enough, backward oscillation will occur since the feedbackmechanism is inherently built in. It is herein noted that the term fastwave tube defines an electron tube in which an electron beam interactswith a circuit wave that has a phase velocity greater than or equal tothe free-space velocity of light. The circuit waves are termed fastwaves and the circuit may be, for example, an unloaded cylindricalwaveguide as shown in FIGS. 1 and 2 or a parallel plane conductor asshown in FIG. 5.

Referring now to FIG. 5 there is shown another em bodiment of thepresent invention. In this instance, a periodic trajectory of theelectron beam is accomplished by using crossed electric and magneticfields. Specifical- 1y, FIG. 5 shows a linear magnetron consisting of acathode plate 31 including an emitter portion 31' located on the lowerhalf near one end of a parallel wire transmission structure 32 formed byspaced anode conductor 34 and cathode conductor 31 and straddling a beamwave interaction area 33 of the linear magnetron. The upper wall 34,opposite from the cathode emitter 3-1, is provided with a positive DCpotential from power supply 40. A collector 36 is provided at the end ofthe magnetron opposite from cathode emitter 31 and insulated therefromby insulators 41. It is noted that the upper and lower walls of parallelwire structure 32 are insulated from each other by side wall portions ofa glass envelope 42. An output loop 37 for supplying the output R.F.energy to the desired load is provided.

During operation, electrons are emitted from cathode emitter 31 andenter into the magnetic field B, into the paper, provided by magnet 38,partially shown, where they .are perturbed into a cycloidal path to theright by the crossed electric and magnetic field provided by magnet 38.Assuming the correct parameters have been obtained between the potentialdifference of the anode, the cathode and magnetic field strength, aportion of the electrons will proceed to the collector in a cycloidalpath.

Considering the R.F. field within the interaction area 33 to propagateas :z the synchronous condition between the beam e and the circuit waveis where E is the DC. potential between the cathode and anode, B is themagnetic lines of force across the DC. potential E and where E is thecharge on the particle and m=the mass of the particle. Therefore,

For low-energy electrons,

so that wE'flB. For pulsed operation, this flux density is notextraordinarily difiicult to obtain. The energy delivered to the RE.circuit field is derived from the transverse kinetic energy of the beam.

The means for phase discrimination for the parallelplate magnetron is asfol-lows. Electrons entering an accelerating R.F. field experience anincrease in transverse energy and return to the lower plate 31 uponcompleting a revolution. Electrons entering a decelerating fieldexperience a decrease in transverse energy and so they remain in the RF.field of the interaction region 33.

What has been shown then are microwave tubes capable of operating in amillimeter waveband employing cumulative interaction between fastcircuit waves and electrons which traverse a periodic path.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a high frequency electron discharge apparatus including, a wavebeam interaction region, means for producing a substantiallyunidirectional beam focusing magnetic field within said interactionregion, means for forming and projecting a stream of electrons into saidmagnetic field with a certain velocity taken in the mean direction ofthe stream and with an initial component of velocity directedtransversely of said unidirectional magnetic field to produce a resonantperiodic trajectory of said electron stream superimposed upon the meandirection of the stream, a two Wire transmission means disposed adjacentsaid electron stream for propagating electromagnetic wave energy at avelocity substantially greator than said certain electron velocity takenin the mean direction of the stream, means for synchronizing theresonant periodic trajectory of said electron stream with saidelectromagnetic wave energy on said transmission means such that acumulative energy transfer from said electron stream to said faster waveoccurs in said WEIIVG- beam interaction region.

2. The high frequency electron apparatus according to claim 1 where saidresonant periodic stream trajectory is a substantially helicaltrajectory.

3. The high frequency electron apparatus according to claim 1 where saidresonant periodic stream trajectory is a substantially cycloidaltrajectory.

References Cited by the Examiner UNITED STATES PATENTS 2,730,648 1/1956Lerbs 313-156 X 2,840,757 6/1958 Dench 315-393 GEORGE N. WESTBY, PrimaryExaminer.

ARTHUR GAUSS, Examiner.

G. R. OFELT, VINCENT LAFRANCHI,

Assistant Examiners.

1. IN A HIGH FREQUENCY ELECTRON DISCHARGE APPARATUS INCLUDING, A WAVEBEAM INTERACTION REGION, MEANS FOR PRODUCING A SUBSTANTIALLYUNIDIRECTIONAL BEAM FOCUSING MAGNETIC FIELD WITHIN SAID INTERACTIONREGION, MEANS FOR FORMING AND PROJECTING A STEAM OF ELECTONS INTO SAIDMAGNETIC FIELD WITH A CERTAIN VELOCITY TAKEN IN THE MEAN DIRECTION OFTHE STREAM AND WITH AN INITIAL COMPONENT OF VELOCITY DIRECTEDTRANSVERSELY OF SAID UNIDIRECTIONAL MAGNETIC FIELD TO PRODUCE A RESONANTPERIODIC TRAJECTORY OF SAID ELECTRONS STREAM SUPERIMPOSED UPON THE MEANDIRECTION OF THE STREAM, A TWO WIRE TRANSMISSION MEANS DISPOSED ADJACENTSAID ELECTRON STREAM FOR PROPAGATING ELECTROMAGNETIC WAVE ENERGY AT AVELOCITY SUBSTANTIALLY GREATER THAN SAID CERTAIN ELECTRON VELOCITY TAKENIN THE MEAN DIRECTION OF THE STREAM, MEANS FOR SYNCHRONIZING THERESONANT PERIODIC TRAJECTORY OF SAID ELECTRON STREAM WITH SAIDELECTROMAGNETIC WAVE ENERGY ON SAID TRANSMISSION MEANS SUCH THAT ACUMULATIVE ENERGY TRANSFER FROM SAID ELECTRON STREAM TO SAID FASTER WAVEACCURS IN SAID WAVEBEAM INTERACTION REGION.